The present invention relates to a cochlear implant. More specifically it relates to a cochlear implant with an increased density and improved quality of electrodes and having physical characteristics which allow it to be placed into a more complete and more effective contact with the cochlea.
Most cases of profound hearing loss may be successfully addressed by a prosthesis that induces the electrical stimulation of the cochlea in response to sound received by a nearby microphone. (The cochlea is an organ of the inner ear, shaped like a snail, in which the auditory neurons have their receptive terminus.) Great efforts have been made over the last thirty years to address profound hearing loss in this manner.
The cochlea includes an electrolyte-solution-filled cavity shaped in the form of a tapering helix, known as the scala tympani. The receptive auditory neurons reside close to the interior or "Modiolar" wall of this cavity. These neurons may be stimulated by the application of electrical potential gradients to this wall.
The range and utility of the prosthetic hearing realized depends on the accuracy and precision with which potential gradients can be applied to this wall. The task of producing potential gradients that exist only within a small selected volume requires many small accurately placed electrodes that may be controlled to work in unison. For example, it may be desirable to control three neighboring electrodes in a cooperative manner as a "triad" to produce desirable potential gradients. The ideal surface area for each electrode is on the order of 1,000 square microns or greater.
In the past it has been typical, due to the unsolved problem of accurate placement of the implant, for each electrode to be in the form of a ring encompassing the implant's lateral circumference. At the current state of the art, these rings could be spaced apart longitudinally by a minimum of 750 .mu.m. With this configuration, regardless of the orientation of the implant, a portion of each electrode faces the modiolar wall.
This configuration of electrodes, however, precludes any manipulation in the lateral dimension of the potential gradients. The potential gradients produced by the ring electrodes decrease monotonically with increasing lateral distance from the electrodes. In contrast, a high density array of electrodes spaced in a grid both laterally and longitudinally can produce precisely shaped lateral gradients with steep, non-monotonic slopes. The auditory neurons, arranged parallel to these lateral gradients, should respond more vigorously to such gradients than to the relatively shallow gradients producible by ring electrodes.
These electrodes must present as low a resistance as is possible to the emission of electrical current from their surfaces. Although the current to be applied is typically very small, current density is significant over the small electrode surface.
Furthermore, each electrode must be resistant to corrosion by the solution that it contacts.
A commonly used method for producing an electrode entails the removal of a small area of insulating dielectric material from a wire, creating an electrode in the form of an exposed wire surface. There are presently several techniques for performing this task, including AC electric corona arcing, direct heating, and plasma etching. These methods have not been completely satisfactory when applied to the biologically compatible dielectric materials which must be used for implants, either because they fail to leave a cleanly and accurately exposed electrode surface, or because the dielectric material forming the rim of the electrode does not adhere satisfactorily and tightly to the wire surface. Mechanical removal of the insulation is very time-consuming and has a high probability of damaging the wire.
Additionally, multiple conductor micro-electrode arrays have been produced using photolithographic-integrated circuit production techniques. These arrays, however, are too delicate for this application. Additionally, such microelectrode arrays lack conductors for connection to other electrical circuitry. The attachment of conductors to such devices, moreover, typically creates a potential failure point.
Techniques that do not involve the accurate removal of insulating dielectric material have also been used to create electrodes for use in a cochlear implant. These techniques are, however, more time and labor intensive than the techniques taught here. Furthermore, it would be virtually impossible to attempt to create an implant with the density of electrodes taught herein using the prior techniques.
Use of red light ruby lasers to pierce dielectric coatings in preparation of microelectrodes was described by M. J. Mela in 1965 in an article entitled "Microperforation with Laser Beam in the Preparation of Microelectrodes," published in IEEE Transactions on Biomedical Engineering, Vol. BME-13, No. 2, pp. 70-76. Unfortunately the use of this type of laser leaves remnants of dielectric coating on the metal surface of the electrode. These remnants interfere with the electrode's ability to emit electrical current. That is, before the present invention it has not been known how to remove biologically compatible dielectric materials cleanly from a metal surface using a laser to produce well-defined, efficient electrodes.
In addition, the helical shape of the cochlea makes it very difficult to place an implant so that it reaches the most remote portion of the scala tympani. Ideally the implant should extend to within 5 mm of the distal terminus of the 25 mm long scala tympani. The preferred location is at the modiolar wall of the scala tympani, which is adjacent to the auditory nerve cells. Pre-shaped cores, shaped insulators with extensions to force the implant to the inner wall, and spiralled implants have all been tried.
The complexity of the shape of the scala tympani, however, a helical structure that turns more tightly at the top and tapers inward, presents a very difficult challenge not yet sufficiently answered. Complicating the problem is the possibility of damage either to the scala tympani or the implant during the insertion process. The techniques heretofore used have generally not proven adequate to allow the safe yet deep insertion of an implant.